The local partiers go to the beaches while the devout stay home and celebrate in their own traditional ways. If you want to skip the beach and see how "real" Nicaragua celebrates the holy week of Easter, I found a great resource that has a run down on what is going on and where: nicaconexiones.com/visit-managua/…
Disclaimer: They are selling tours. BUT the information on the page is invaluable for any independent traveler who wants to set out on their own for the week. Even though I sense it is a shortened version of events, it is too long to cut and paste for all to see here.
Any other events going on for Semana Santa?